In recent years, injection molding has been widely employed for molding an acrylonitrile-butadiene rubber (hereinafter referred to as "NBR") into articles in view of productivity and rationality, for which good oil resistance and heat resistance are required. Recently such articles broadly include, for example, rubber vibration insulators, electrical parts, automobile parts, industrial parts and footwear.
It is required for the injection molding of NBR that NBR has a good flowability and, when NBR is vulcanized at a high temperature for a short time, the vulcanization is effected at a high efficiency. Namely, a high-rate vulcanizability is required. In general, the higher the vulcanization temperature, the lower the degree of vulcanization of rubber. Thus injection molded articles have poorer tensile stress and impact resilience than those of compression molded articles (for example, Journal of the Society of Rubber Industry, Japan, vol. 59, No. 4, p 214-215, 1986).
To obtain a high rate of vulcanization in the injection molding of NBR, many methods have been proposed which include, for example, a method of introducing a functional group such as a carboxyl group or an amino group into the molecule of NBR, a method of incorporating a special vulcanization promotor, and a method of, when an emulsion polymerization is conducted for the production of NBR, using minimum amounts of an emulsifier and a coagulant so that minimum amounts of the emulsifier and the coagulant remain in the resulting NBR. These proposed methods have problems such that the enhancement of the rate of vulcanization is not sufficient and some other characteristics such as cold resistance and compression set are deteriorated.
Further, the contamination of a mold is conspicuous in the injection molding of NBR wherein vulcanization is carried out at a high temperature. Namely, when the molding of NBR is repeated, staining materials are gradually deposited on the mold with the result of contamination of NBR moldings and spoiling of the surface appearance. Therefore the mold must be cleaned at regular intervals. The cleaning is time-consuming and costly and leads to reduction in productivity.
To avoid the contamination of a mold, an attempt has been made wherein talc, sodium thiosulfate, carbon wax or silicone oil is incorporated in NBR. However, even when this attempt is made for injection molding of commercially available general-purpose NBR wherein vulcanization is carried out at a high temperature and a high rate, the contamination can be avoided only to a limited extent.